It can get cold in Hawaii.  I know, I know.  I've been hearing it forever:

  • "My year in Hawaii we lived happily with no heat..."
  • "Winter is Not Coming"
  • "What do you mean, it's cold?  You're in Hawaii!"
  • "Do you even NEED walls?"

I live on the wet side of Waimea, north of Mauna Kea.  You know what "mauna kea" means, right?  It means "white mountain."  You know why it's white, right?  Because it snows sometimes.  Today's news:

The last one is interesting.  It's an "unseasonal" snowstorm.  Is it global weirding again?

So houses need heat.  Not all houses in Hawaii...if you live in Kailua-Kona, or Hilo, or Honolulu, you don't need heat.  But some of us do, and how big a heating system?  Buying a heating system is like buying shave ice.  If you get the toddler size, you eat it up and are left wanting more.  You shoulda bought the big one, brah.  But if you get a bucket, you probably won't be able to eat it all before it melts, so it's a waste.

If a heating system is too small, it can't keep up.  If it's too big, it's a waste of your money.  To right-size a heating system, I do heat loss calculations.  To do those, I collect data & measurements of the house, put it all in a model, and grab climate data from somewhere.  My software has some weather stations for me to choose from, but there's a problem.  These are the Hawaiian weather stations available in my software.  I put the average annual Heating Degree Days (HDD) for each site next to them:

  • Barbers Point Naval Air Station (1 HDD)
  • Hilo International Airport (4 HDD)
  • Honolulu International Airport (0 HDD)
  • Kahului Airport (0 HDD)
  • Kaneohe Bay Marine Corps Air Station (0 HDD)
  • Kapalua (0 HDD)
  • Kona International Airport (0 HDD)
  • Lanai (16 HDD)
  • Lihue Airport (1 HDD)
  • Molokai (Amos) (4 HDD)

HDD are a measure of how much energy it will take to heat a house - the more HDD, the more energy.  For comparison, Barrow, Alaska has more than 19,000 HDD.  Minneapolis has more than 7000.  I live in Waimea on the Big Island and I can see the snow on the mountain from my house, so obviously none of these weather stations will give me good data to work with.

After some digging, I found the Hawaii State Climate Office, which put together some good numbers I can work with: 

Those are average numbers of HDD per month, Jan-Dec.  Depending on where you are in Waimea, there are 600-800 annual HDD.  That's quite different from Honolulu or Hilo.  After some MORE digging, I found a place that has 800 HDD and a weather station my software can use, and it's...Daytona Beach.

I mentioned this to my mom, who used to live in Florida, and she said, "Oh yes, it gets very chilly in Daytona."

People need to be warm and dry, because if they're cold and damp they get sick.  The mold sets in.  Indoor air quality takes a nosedive, because you're inside more and you closed up your house.

Next up, part 2: how to choose a heating system that you'll be happy with for years to come, that does just what you need.  And what if it could ALSO give you air conditioning?  And dehumidify your house?

Are you looking for help with a heating system?  Let me help.  I'll collect data, we'll talk about what you need, and figure out what's not too big, not too small, but juuuuust right - the Goldilocks Approach.  You'll probably end up with something smaller than you thought, because bigger is not better.  It'll also run more efficiently, saving you energy.

For more on Hawaii Island's double-handful of climate zones, go here:

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Tags: climate, energy, hawaii, heating


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Comment by Blake Reid on December 12, 2017 at 12:44pm

Yes, all of that.  The next part (stay tuned for part 2) will be about my hunt for a decent design temp.  I found a Weather Underground station very near me, and in looking at it, I'm in the 52-55 range.

I do like it when I get to come at a problem from a couple different directions.  Part of my issue with TREAT is that you don't know what's going on behind the curtain.

It'll be a heat pump, even with HI's highest-rates-in-the-country.  It's not like it'll use THAT much, I mean, it's still Hawaii, right?  ;)  Some people have woodstoves.  Very few, if any, ducted things.  Propane is an option.  But with a heat pump, I'll get AC too.  I'm much less concerned about the sizing of the AC, since it's air conditioned all the time up here.  So I'll have another thing I won't use very much!

Comment by David Butler on December 11, 2017 at 10:08pm

I haven't used Treat but if it has a Design Heating and Cooling load report, I'm sure it's based on the design temperatures. And since it has the TMY3 hourly climate files, it can easily calculate the design temperature for any TMY3 location.

I misunderstood. I understood that your software used HDD to calculate design loads. You must have been using HDD as a proxy for climate to find a comparable TMY3 location. In my experience, HDD is a poor proxy for this purpose. For example, even though Daytona has similar HDD's as your location, it has a mean extreme of 39F and an *average* January low of 47F. Even Miami, which only has 128 HDD has a mean extreme of 41F, colder than you indicated for your location. If the climate office has average monthly lows/highs, that's a better proxy when comparing to other locations. Otherwise, based on your comment that the coldest it gets is upper 40's, I would estimate your design temperature is in the low-to-mid-50's.

In any case, in the case of heating, there's not much downside if the unit ends up a bit oversized. What kind of heating system to you intend to install? Normally I would recommend a heat pump but I understand Hawaii's electric rates are sky high!

Comment by Blake Reid on December 11, 2017 at 2:55pm

David, you've found my problem.  How to find good inputs, still use some "judgment," and feel good about coming up with a decent solution.

I use TREAT.  I don't like TREAT, but it has a couple of advantages for me: (1) I own it, and (2) I don't like it, but I'm used to it.  An available report is the "Design Heating and Cooling Load".  It "presents results of the load sizing calculations.  TREAT load sizing has been tested...and results were compared to Manual J.  TREAT heating and cooling loads proved to be slightly more conservative."  TREAT's using TMY3 data from Daytona for this project.

Along the way, I'm figuring out what my design temp is.  Coldest it's been inside is 48 degrees so far.  (It's basically the same outside.)  And then I'll look at what TREAT says my heating system needs to be and probably downsize my heat pump one notch.  ;)

Comment by David Butler on December 11, 2017 at 2:28pm

What software are you using? Proper heat load calculations are based on the local Design Temperature, not HDD as degree days tell us nothing about how much capacity is needed to heat or cool a home. Degree days can be used to roughly estimate *annual* heat or cooling loads.

Load calc design temperatures are statistical, based on "TMY" (typical meteorological year) hourly datasets. In most cases, the 99 percentile value is used for residential heat loads, i.e., the temperature above which represents 99% of the hours in the year. For example, if the design temperature is 40F, that means 99% of the hours, historically, fall above 40F and 1% fall below.

When a heating or cooling system is sized to the 99% design temperature, that doesn't mean the home will be uncomfortable during the remaining 1% of the time. That's because the coldest 1% of the hours usually only persist for a short time, an hour or two before sunrise. The home's mass and insulation create thermal lag which significantly dampens the response at the extremes.

In cases like yours where there's no reporting station nearby, you can estimate the design temperature based on non-TMY climate data or your own experience living in the area. In mild climates like Florida, the heat design temperature is typically around 10 to 12 degrees above the mean extreme temperature. For example, Daytona has a mean extreme temperature of 28F and a design temperature of 39F.

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